This invention relates generally to the transmission of data to multiple users, and more particularly the invention relates to a power efficient method of simultaneously transmitting a plurality of data packets to multiple users.
In a star configured data network topology, the downstream transmission of data is emitted from the base station, located at the hub of the star, to the subscriber stations located at the points of the star. One of the physical characteristics of the downstream transmission that is often constrained is the emitted power, either average or peak or both. These constraints are usually derived from regulatory limits, transmission equipment capabilities, or operator cooperative agreements.
The very nature of a network suggests that the data transmitted to the subscriber stations must share the transmission medium resources. One of the critical resources used in the downstream transmission is the total power of the transmitter. In time division multiple access (TDMA) each user is assigned a time slot during which full power can be devoted for transmitting to one user. This is not power efficient since some users require less than full available power. In frequency division multiple access (FDM), each user has full-time use of a limited portion of the transmission bandwidth.
Orthogonal direct sequence spread spectrum or orthogonal code division multiple access (OCDMA) allows each user to use the full bandwidth full time. In this transmission mode, a plurality of data packets is assembled for coincident transmission. Each data packet is directly spread by a separate orthogonal code sequence that is assigned to the recipient, thus creating a transmission burst associated with each data packet. Further, each transmission burst is assigned a power level that will insure proper reception at the intended subscriber station. All power scaled transmission bursts are combined to form a composite burst that is sent out as a downstream transmission.
Given this manner of downstream transmission generation, there is a difficulty encountered when some form of power limitation exists. To conform to the power limit, the base station may reduce power proportionally for all transmission bursts in the composite or selectively eliminate some transmission bursts to reduce the composite power and thereby meet the limit. In the case of a proportional reduction of all transmission bursts, the link performance requirement may not be met and the linked users can experience unacceptable error rates. In the other case where selective elimination occurs, those transmission bursts included in the composite have data packet integrity, but those selected for elimination are lost. In either case the capacity of a network is limited due to the loss of data integrity at the receive end or the loss of entire packets at the transmit end.